<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title>固定定位</title>
    <style>
        .nav{
            width: 500px;
            height: 600px;
            border: 1px solid red;
            float: left;
            position: fixed;
        }
        .main{
            width: 500px;
            height: 1600px;
            border: 1px solid aquamarine;
            float: right;
        }
    </style>
</head>
<body>
    <div class="nav">
        这是左边导航
    </div>
    <div class="main">
        这是右边主要区域
    </div>
</body>
</html>